1. Field of the Invention
This invention relates generally to a powertrain for a hybrid electric vehicle (HEV) and, more particularly, to a powertrain with a dry dual-clutch automatic transmission and its control during an engine restart event.
2. Description of the Prior Art
A hybrid electric vehicle (HEV) is a vehicle configured with a hybrid propulsion system that utilizes at least two different sources of torque for propelling the vehicle. A hybrid propulsion system may combine a conventional propulsion system that includes an internal combustion engine and a stepped-ratio automatic transmission with an electric propulsion system that includes one or more electric motors and a rechargeable energy storage device, such as a battery, that can power the electric motors or store energy to improve fuel economy over the conventional vehicle. A hybrid electric vehicle typically provides different powertrain operating modes with the engine running or shutdown depending on the vehicle operating conditions, battery conditions, and driver's propulsion request. Hence, one of the major functions that an HEV provides is the ability to start or stop the engine during certain conditions. When the engine is running, the electric portion of the propulsion system may be used to assist the engine in providing the required vehicle propulsion. During the conditions when the engine is shutdown, the driver's propulsion request can be provided entirely by the electric motor.
Motor vehicles can be designed to employ certain aspects of hybrid electric technology to reduce fuel consumption, but without use of a hybrid drivetrain. In such vehicles, called “micro-HEVs”, shutting down the engine during conditions where the engine operates at idle speed will be used to reduce fuel consumption and reduce emissions in a conventional powertrain that includes an internal combustion engine and a stepped-ratio automatic transmission, but no electric machine for driving the wheels. In a micro-HEV with an automatic transmission, the primary condition that is checked by the powertrain control system before stopping the engine is that the driver has applied the brakes and the vehicle is stopped since the engine would typically be idling during these conditions in a conventional vehicle. Once the driver releases the brake pedal indicating a request for vehicle propulsion, the powertrain control system will automatically restart the engine.
In a Micro-HEV application with a dry dual-clutch automatic transmission, coordinated transmission control before, during, and after an engine start is critical to acceptable vehicle performance. Specifically, the control of transmission clutch torque capacity during the engine start directly controls the amount of engine torque transferred to the wheels for vehicle propulsion. Moreover, any delays in clutch torque capacity application directly result in non-responsive vehicle propulsion and poor vehicle performance. Furthermore, in vehicle applications using a dual-clutch (powershift) automatic transmission without a torque converter, any excessive clutch torque capacity during an engine restart can lead to engine stall and/or rough creep and launch performance.
One method commonly applied to address these issues is simply keeping the transmission in a neutral state during engine shutdown and restarts. Once the engine is running, the transmission clutch torque can be increased so that torque is transmitted to the wheels. A problem with this approach is that poor vehicle response will be perceived by the driver since vehicle propulsion is not provided until the engine has been started and clutch torque capacity has been increased. In conventional and wet-clutch dual-clutch automatic transmission applications which are electro-hydraulically actuated through a pump mechanically driven by the engine, an electric auxiliary pump would be used to provide hydraulic pressure for any clutch actuation while the engine is not running.
A dry dual-clutch automatic transmission is an automatic transmission whose clutches' torque transmitting capacity varies in response to electro-mechanical actuation rather than by pressurized hydraulic fluid.
A strategy is needed to coordinate transmission and engine control during an engine start event while minimizing energy consumption in order to provide responsive, smooth, consistent and predictable vehicle propulsion performance.